1. Field of the Invention
The disclosures herein generally relate to a heater substrate, an alkali metal cell unit and an atomic oscillator.
2. Description of the Related Art
An atomic clock (an atomic oscillator) is an extremely accurate clock. Technologies for reducing the size of the atomic clock are being studied. The atomic clock is an oscillator based on transition energy of an electron included in an atom of alkali metal or the like. Especially, electron transition energy of an alkali metal atom is found to be quite accurate if the process is not influenced by any disturbance. Accordingly, stability of frequency, which has been improved by more than several orders of magnitude compared with a crystal oscillator, can be obtained.
There have been several types of atomic oscillators. Among such atomic oscillators, a CPT (Coherent Population Trapping) type atomic oscillator provides stability of frequency, precision of which is higher by three orders of magnitude than that of the crystal oscillator of the related art. Furthermore, it is hoped that size and power consumption of the CPT type atomic oscillator can be reduced (See for example, Appl. Phys. Lett. Vol. 85, pp. 1460-1462 (2004) and Comprehensive Microsystems, Vol. 3, pp. 571-612).
The CPT type atomic oscillator includes, as shown in FIG. 1, a light source 910 such as a laser element, an alkali metal cell 940 in which alkali metals are encapsulated, and a light detector 950 which receives a laser light that passes through the alkali metal cell 940. The laser light is modulated, and by sideband wavelengths appearing on both sides of a carrier wave which are specific wavelengths, excites electrons in alkali metal atoms with two simultaneous transitions of the electrons. Transition energy of the above transition is constant. When the sideband wavelength coincides with a wavelength corresponding to the transition energy, a transparency phenomenon that absorption of light by the alkali metal decreases occurs. In this way, the CPT type atomic oscillator is an atomic oscillator having a feature that while a wavelength of the carrier wave is adjusted so that the absorption of light by the atomic metal decreases, a signal detected by the light detector 950 is fed back to a modulator 960, and a modulation frequency of the laser light from the light source 910 such as a laser element is adjusted by the modulator 960. Meanwhile, the laser light is emitted from the light source 910 and is transmitted onto the alkali metal cell 940 via a collimator lens 920 and a λ/4 plate 930.
A manufacturing method for the alkali metal cell in the above-described compact atomic oscillator by using a MEMS (Micro Electro Mechanical Systems) technique is disclosed (U.S. Pat. No. 6,806,784, US Patent Application Publication No. 2005/0007118, Japanese Published Patent Applications No. 2009-212416, and 2009-283526). In the method disclosed in them, at first, an aperture is formed in a silicon (Si) substrate by using a photolithography technique and an etching technique, and then the Si substrate is bonded to a glass plate by an anodic bonding. The anodic bonding is performed at a temperature of from 200 to 400° C., applying voltage of about from 250 to 1000 V to an interface between the glass and the Si substrate. Afterward, an alkali metal and buffer gas are input, and the cell is sealed by binding the glass to an aperture part which is a top side by the anodic bonding. By cutting out what is formed in this way for each cell, an alkali metal cell is formed.
The alkali metal cell, as described above, is also called an alkali metal gas cell. In order to make the sealed alkali metal and buffer gas be in the form of gas, the gas cell is heated to the predetermined temperature. For example, a heat body configured with a transparent conductive film such as Indium Tin Oxide (ITO) is provided on a surface of an alkali metal cell in an atomic oscillator. An electric current is applied to the heat body to make the heat body produce heat. In this way, the alkali metal cell with the heat body provided on the alkali metal cell may be called in the present application an alkali metal cell unit.
The atomic oscillator is constantly controlled by feedback so that temperature in the alkali metal cell is kept constant. Therefore, when outside temperature changes, the current flowing through the heat body is also changed. In this way, when the current flowing through the heat body changes, a magnetic field generated by the heat body also changes. The generated magnetic field raises a problem in that a frequency corresponding to an energy difference between ground levels of a metallic atom in the alkali metal cell varies and an output frequency may be shifted. Accordingly, it is desirable that an unnecessary magnetic field generated by a heater be suppressed as much as possible in the atomic oscillator.
As a technique of eliminating such an unnecessary magnetic field generated by a heater, Japanese Published Patent Application No. 2012-191138 discloses a method of forming a heater having a meandering pattern of ITO or the like. Moreover, Japanese Published Patent Application No. 2010-71973 discloses a method of forming a heater having a winding pattern of a metallic material or the like.
In the methods disclosed in Japanese Published Patent Applications No. 2012-191138 and No. 2010-71973, a wiring is formed in the meandering pattern or the winding pattern. Directions of electric currents flowing in adjacent wirings are opposite to each other, and opposing generated magnetic fields cancel out each other, so that the effective magnetic field is weakened. In the method disclosed in Japanese Published Patent Application No. 2012-191138, it is necessary to use a transparent conducting material with a high resistance, and a range of choice of a material for forming the heater is narrow. On the other hand, in the method disclosed in Japanese Published Patent Application No. 2010-71973, for the material forming the heater, a metallic material with a low resistance can be used.
High accuracy is required for the atomic oscillator, and the weaker the magnetic field generated by applying a current to the heater is, the more desirable it is. However, in the method described in Japanese Published Patent Application No. 2010-71973, the magnetic field generated by applying current to the heater has a lower limit, and the magnetic field cannot be made sufficiently weak.